The present disclosure relates to bonded abrasive articles such as grinding wheels and the like. More particularly, it relates to methods of making bonded abrasive articles, including formation of pores in the resultant bonded abrasive article.
Bonded abrasive articles useful for abrasive machining and other operations are three-dimensional in structure and typically include a shaped mass of abrasive particles bonded together by a binder. Such shaped mass can be, for example, in the form of a cylinder or wheel, such as a grinding wheel, cut-off wheel, etc. The main types of bonding systems used to make bonded abrasive articles are: vitrified, resinoid, and metal. With vitrified bonded abrasive articles, such as vitrified grinding wheels, a vitreous binder medium or system is used to bond the abrasive particles together. These bonds are usually vitrified at temperatures between 700° C. to 1500° C. Resinoid bonded abrasives utilize an organic binder system (e.g., phenolic binder systems) to bond the abrasive particles together to form the shaped mass. Metal bonded abrasive articles typically utilize sintered or plated metal to bond the abrasive particles. Vitrified bonded abrasive articles are different from resinoid bonded abrasive particles in that they use a vitreous phase to bond the abrasive grains and thus are processed at substantially higher temperatures. Vitrified bonded abrasive articles can withstand higher temperatures in use and are generally more rigid and brittle than resinoid bonded wheels. Vitrified bonded abrasive articles are well-suited for precision machining.
It has been found that the cutting characteristics of bonded abrasive articles, such as vitrified bonded grinding wheels, can benefit from the inclusion or formation of pores. Bonded abrasive articles containing or exhibiting porosity have an open structure (interlinked or interconnected porosity) that can provide chip clearance for high material removal and can transport more coolant into the contact area while decreasing friction. Absent adequate porosity and spacing between abrasive grains, the bonded abrasive article can become loaded with chips and cease to cut as expected. Also, porosity can facilitate a self-sharpening attribute, enabling a bonded abrasive article to shed used or worn abrasive particles to expose new cutting edges or fresh abrasive particles.
A minimal level of porosity may naturally occur when packing or molding the abrasive particles and binder medium under pressure. However, this natural porosity is typically insufficient for many vitrified bonded abrasive article end-use applications. As such, pore inducing components or additives are included with the vitreous bonding abrasive composition or mixture. Pore inducing components or additives can be non-temporary (i.e., components present in the abrasive composition and in the final bonded abrasive article), temporary (i.e., components present in the abrasive composition but not present in the final bonded abrasive article), and combinations thereof. Typical non-temporary pore inducing components include hollow glass spheres and hollow ceramic spheres. Typical temporary pore inducing components include cork, ground walnut shells, wood particles, and polymeric materials. These temporary pore inducing components are burnt out of the abrasive matrix as part of the vitrified abrasive bonded article manufacturing process.
While ground shells, wood particles, and the like are viable temporary pore inducing materials, but tend to have variability in supply and raise concerns with open fires with the kiln since the material does not sublime. Many polymeric materials are elastic in nature and experience spring-back that in turn can make it difficult to predict or precisely control a geometry of the resultant vitrified bonded abrasive article after firing. Naphthalene is another well-known temporary pore inducing material that does not present these same spring-back concerns. While more commonly known as the main ingredient of traditional moth balls, the physical and material properties of naphthalene are well-suited to the vitrified abrasive bonded article methods of manufacture. Naphthalene can be readily shaped to create pores of a desired size, does not react with conventional vitreous bonding materials or abrasive grit, and exhibits little or no volume change under expected pressures or pressing conditions. Further, naphthalene readily sublimates at the temperatures (and pressures) of the vitrified bonding process, and thus is cleanly removed from the resultant bonded abrasive article. However, naphthalene is viewed as possibly carcinogenic to humans and animals, and is not pleasant to work with. Moreover, various governmental agencies have established (or are considering) rigorous naphthalene exposure limits that may effectively render it un-usable for the mass production of vitrified bonded abrasive articles.
In light of the above, a need exists for methods of manufacturing bonded abrasive articles, such as vitrified bonded abrasive articles, with porosity induced by environmentally benign materials otherwise exhibiting the process control attributes of naphthalene.